centimorgan

A unit of genetic distance, used to describe the separation of genes on a
chromosome. It dates from the early 20th century, the days of genetic research with fruit flies, before the discovery
of DNA and the mapping of genomes using restriction enzymes and other advanced
techniques. The terms “millimorgan”
and "sub-millimorgan" are also in use (but not micromorgan). The unit
originated in work by Thomas Hunt Morgan in 19 and was refined and named by
J. B. S. Haldane in 1919.

One centimorgan is defined as the genetic distance between two loci with a statistically corrected recombination frequency
of 1%; the genetic distance in centimorgans is numerically equal to the recombination frequency expressed as a percentage.
Symbol, cM. The centimorgan is now
more commonly called a “map unit” (symbol, mu) or locus map unit (symbol,
LMU).

The qualification “statistically corrected” is necessary because at
genetic distances greater than about 7 cM, the relationship between
recombination frequency and genetic distance is no longer linear. Researchers
have developed mathematical models that can correct for this difficulty.

Other mapping functions are possible. After Haldane's, the most common is
that of D. D. Kosambi. Haldane function is purely mathematical; Kosambi's
incorporates an empircial term. distinguish between "Haldane
centimorgans" or "Kosambi centimorgans"

The centimorgan is not a measure of physical distance, but typically a genetic distance of 1 cM corresponds to
a physical distance of roughly one million base pairs. Attempts to assign a physical length to the centimorgan have led to an
estimate that it is roughly about 0.003 millimeters.

History

In 1911, A. H. Sturtevant, an undergraduate working in the geneticist T.
H. Morgan's laboratory, had an inspiration: a map of a chromosome showing relative locations of
genes could be made using the phenomenon of "crossing over." During
meiosis, two homologous chromosomes may link, so that one in each new pair
of chromosomes combines part of a chromosome from the mother with part of the father's.
If
the break takes place between two genes, a new recombinant gamete is formed,
which may be detectable in the appearance of the new organism. The
farther apart the two genes are on the chromosome, the more likely it is that they
will recombine.